A 12,000 YEAR TEMPERATURE RECORD OF THE NORTH AMERICAN GREAT PLAINS BASED ON THE STABLE CARBON ISOTOPIC COMPOSITION OF PALEOSOL ORGANIC MATTER

Recent data collected from 73 native grassland soils throughout the North American Great Plains show a strong inverse relationship between latitude and stable C isotopes of organic matter and a strong direct relationship between stable C isotopes of organic matter and mean July temperature. Consequently, stable C isotopes from buried paleosol organic matter should be a reliable proxy for soil biomass production from C4 plants and temperatures in the past. A literature compilation of 202 stable C isotope values from radiocarbon-dated buried paleosols in fluvial and eolian environments was analyzed for the last 12,000 years from Texas (29.5°N) to North Dakota (46.5°N) to establish latitudinal trends. A second-order polynomial regression equation was used to transfer stable C isotope values to mean July temperatures. C4 soil biomass production from 12-11 ka indicates that temperatures were 2 to 3°C cooler than present at all latitudes. During the Younger Dryas, C4 production points to a temperature increase of approximately 1°C at all latitudes. From 11-7 ka C4 production and temperatures stabilized at Younger Dryas levels. However, from 7-5 ka C4 production levels transfer to a temperature increase of nearly 2°C, approximating the modern stable carbon isotope-temperature curve in both magnitude and shape at all latitudes. Slight variations in C4 production and temperature occurred during the last 5 ka, but were maintained near-modern levels. Throughout the North American Great Plains, fossil pollen data suggests that relative to present temperatures were -3 to -6°C at 12 ka, –2 to 0°C at 9 ka, and 0 to +2°C at 6 ka. All temperature estimates using the C4 transfer function in this study fall within the range of pollen estimates. Importantly, a cooling episode associated with the Young Dryas was not recognized, and maximum C4 production occurred 2000 years after attainment of maximum summer solar insolation at 9 ka. In contrast to current thinking, flourishing of C4 plants during the middle Holocene took place during a rapid rise in atmospheric CO2 from 220 to 280 ppmV. A new C4 soil biomass transfer function for temperature provides a much needed climate proxy that can be applied more pervasively throughout the Great Plains than fossil pollen.